| 摘要 |
PURPOSE:To prevent malfunction caused by the disturbance of a transmission system by releasing the maintenance of loopback constitution after plural periodic changes in a hold signal are eliminated. CONSTITUTION:A control section 23 of an optical loop system 5 has functions sending branched light at the sending side from the loop system 5 by a loopback instruction, detecting the abnormality of light and informing it to the outside. A control section 26 of an optical loop system 6 has similar functions. Hold signal recognition control sections 24, 27 detect a hold signal 7 sent from a central section station and gives a release instruction of the maintenance of loopback constitution after plural consecutive changes are detected. The hold signal 7 for maintaining the loopback constitution is generated from the central station and the loopback is maintained while the signal 7 exists. In releasing the loopback, the hold signal 7 is plurally changed in the central station.
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